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Bonding Agent as an Instrument Lubricant

Potential Effect on Marginal Integrity

William H. Liebenberg, BSc, BDS (Rand)

The ease of handling and placing amalgam has often discouraged clinicians from the use of composite materials for the direct restoration of posterior lesions. Nevertheless, a discriminating shift has occurred as amalgam protagonists have begun to explore contemporary composites with more favorable viscosities. This material conversion is due in part to the marketing efforts of manufacturers who have launched new composite formulations that purportedly facilitate the clinical "feel" of amalgam and allow for the employment of amalgam-type restorative procedures (ie, "condensing" to achieve adequate interproximal contact). Enhancements in the chemistry of the resin matrix, filler content, and particle size distribution have resulted in the development of several higher-viscosity posterior composite materials with impressive physical properties.

Substantial variance in handling exists between the "regular" viscosity posterior composites and the higher-viscosity formulations. Since increased filler content has largely precluded cartridge delivery, the majority of the higher-viscosity resin materials designed for the posterior dentition are packaged in syringes. This manner of delivery requires extrusion, excision from the resin mass, transport to the cavity, and tailored adaptation to the internal aspect of the cavity form. In the author’s experience, the higher-viscosity resin formulations exhibit no flow and require increased manipulation, which results in extended instrument/resin interaction. In addition, the stickiness (also referred to as "tackiness" or "pull back") of the resin to the instrument can become a nuisance in clinical practice. The reality--irrespective of what composite is used--is that the material will be inclined to adhere to a slightly flawed or soiled instrument, which results in withdrawal of the composite material from the cavity as the instrument is removed.

The solution to this annoyance in clinical practiceis to wipe the instrument with a resin-soaked gauze swab. Time and financial restraints soon warp the intentions of this lubricating practice: the wipe becomes the dip, and lubrication turns to saturation. Why is this a problem with posterior composites? Consider the following case study.

Case Presentation

A 37-year-old patient presents for the direct aesthetic restoration of the maxillary first molar; the existing amalgam restoration is defective along the occlusal margin with enamel infractions. The initial dilemma faced by the clinician is the selection of the appropriate posterior composite material. In an attempt to overcome a deficiency of long-term clinical trials with recent composite materials, manufacturers have partnered with universities to verify physical properties through in vitro scientific studies. Product profiles are increasingly scrutinized by enlightened clinicians for wear, volumetric shrinkage, flexural moduli, and compressive strengths. Other than resistance to fracture, however, many physical properties are of "esoteric" interest in clinical practice, as it is difficult to evaluate the physical performance of one particular brand of composite against a competitive product--particularly when consensus is sought between conflicting studies. Conversely, handling is subjective and rigorously appraised at chairside. As a consequence, clinicians are inclined to rank composite formulations according to ease of placement and submission to sculpting manipulation (albeit with the qualifying codicil of "in my hands").

Although it is impossible to single out one aspect of our restorative goal, marginal integrity must rank as a priority when evaluating restorative success, since an intact restorative/tooth interface is crucial to the exclusion of bacteria and the hydrodynamic equilibrium of the dentino-pulpal complex. In the accompanying case study (Figures 1-2-3-4-5), astute clinicians will observe the adherence to fundamental operative adhesive guidelines as the objective of marginal integrity is pursued. It may be a small detail, yet the presence of composite residue - visible on the uppermost portion of the instrument depicted in Figure 3 - could ultimately compromise the integrity of the composite/tooth interface.

Low-modulus flowable composite formulations that exhibit inordinately high shrinkage will not provide long-term marginal integrity on occlusal load-bearing surfaces. Accordingly, their use is contraindicated in the occlusal aspects of Class I and Class II situations. Yet clinicians in search of improved handling (less sticky under clinical conditions) have as a solution resorted to modifying the material by inadvertently incorporating resin into the material content, which decreases the modulus and increases the flex at the critical interfacial margins. This practice may be incorporated from procedures used to perform direct composite restorations in the anterior region. The saturation is increased in extensive restorations, which increases the deleterious potential of modifying the physical properties of the resin. In essence, a new flowable resin is formulated in situ as the initial intent of lubrication becomes one of resin saturation and composite dilution.

Equally disturbing is that instrument lubrication is often performed with the same one-bottle dentin-enamel bonding agent (DBA) utilized in the primary bonding procedure. Consequently, solvent is being applied to the composite at the restorative interface. Unlike the initial DBA application, air-assisted evaporation of the solvent is not encouraged, which could have consequences for the restorative interface given that previous research has demonstrated adhesion to be compromised when solvents are not evaporated properly prior to polymerization.1,2

Conclusion

In summary, if a material has been selected on the merit of its physical properties, then it is the clinician's responsibility to ensure that the integrity of the material is not compromised during manipulation. Numerous variables (eg, material shrinkage, occlusal load) that are beyond the control of the clinician can affect the integrity of the bond between the composite and the tooth substrate. While the composite resin is selected with these challenges in mind, it is important to remember that the physical properties of the material relate to the unaltered product. Composite dilution is an ubiquitous procedural error, as an instrument that is saturated with resin prior to composite manipulation allows for seemingly deft adaptation along the cavosurface margins. While the handling and short-term marginal integrity of a rigid high-viscosity composite material appears to be improved by the addition of an unfilled resin, the inevitable marginal deterioration may be a product of material abuse rather than material failure. The dilution of the composite may jeopardize the success of direct composite posterior restorations, and the solution may be as simple as "wipe, don't dip".